In diseases such as cancer, it is known that the condition of the disease is correlated with the hardness of the lesion. Knowing a hardness distribution within a living body contributes to very significant information in its diagnosis. One method of quantitatively obtaining the hardness of a tissue is a method that obtains an elasticity distribution from the velocity of shear wave propagation. In this method, a shear wave is generated within a living body, and the shear wave is measured using ultrasound, thereby estimating the propagation velocity.
As a method of generating a shear wave, a method of pressing a tissue in a living body using acoustic radiation pressure generated by emitting ultrasound has been recently proposed (for example, see NPL 1).
In the method described in NPL 1, an ultrasonic burst wave (push ultrasonic wave) of approximately hundreds of microseconds, for generating a shear wave, is emitted, and pressure (hereinafter may also be referred to as vibration) is applied using acoustic radiation. Thereafter, to observe the propagation of the shear wave generated within a target object, ultrasonic image measurements are performed at a recurrence frequency of about a few kHz (track ultrasonic wave). Observation using a track ultrasonic wave is generally performed for a duration of about a few ms to tens of ms. From the obtained image, a displacement distribution is measured, and the velocity of the shear wave propagation is estimated.